Among urban regeneration-related debates, many practitioners and scholars tend to see some physical and nonphysical interventions as stimuli or catalysts. Thus, the projects having broader effects than their specified sites have strategically been designed to regenerate the urban decay in an area. The concepts of prestige projects, flagships, and pioneer projects have been mainly raised with this prime purpose (Smyth, 1994). The concept, developed under catalytic theory, implies that one or a series of elements can impel and guide subsequent developments and lead to the incremental, continuous regeneration of the urban fabric (Attoe & Logan, 1989). More recently, a new related term, urban acupuncture, has been introduced as small interventions replacing grand plans and redevelopment schemes (Madeddu, 2016) and positively affect social, ecological, and even economic and physical dimensions of the surrounding environment (Daugėlaitė & Gražulevičiūtė-Vileniškė, 2018).

A wide range of physical and non-physical interventions are considered as stimuli or catalysts in the related literature. From those notable large-scale and expensive physical developments called megaprojects and prestige projects to the smaller ones called flagships and further to the trivial physical and non-physical changes leading to a gradual transformation of an urban area can be called catalysts. Undoubtedly, there are different views on each type of catalyst. Many scholars have addressed the success and failure stories of the relevant experiences ((Attoe & Logan, 1989); (Loftman & Nevin, 1995)). However, the main idea of catalysts has remained attractive and valuable. Who is not attracted to the anecdote of the positive metamorphosis of a left, decayed, and undesirable urban area as a result of the development of a museum or the installation of a fountain only?

It is noteworthy to say that the idea of utilizing urban catalysts refers to the existence of a situation of need to change and a call for transformation stemming from the abandoned, decayed, poor, unsafe, unstable, and undesirable urban areas. In short, the catalysts encounter the problematic urban regions, which already relate to several tensions, service and infrastructure lack, social discontent, and poor quality. However, some of the projects could not create values and regenerate their peripheral areas and even have led to negative consequences and empty promises (Loftman & Nevin, 1995). The critics matter more when the projects and their developers have gotten a considerable amount of public budget, funds, loans, privileges supporting a project with catalytic benefits for a larger group of people.

Nearly all projects, especially the large-scale and index ones, lead to broad impacts on their surroundings in terms of physical aspect (changing the main structure of the city or neighborhood), landscape, traffic (increasing the vehicle and pedestrian trips demands), social and economic status of an area. So, the idea of catalyst projects wants to emphasize the positive and long-term impacts. Therefore, to have suitable catalytic projects, it is crucial to ensure and boost the value of the project's results and control their negative consequences.

According to the Statistical Center of Iran, the City of Tehran, the capital and the most populated city of Iran (over 8 million population in 2017, according to the Statistical Center of Iran), has experienced rapid population growth and physical expansion over the last decades (after the 1970s). The growth is accompanied by the dilapidation or rundown of neighborhoods suffering from poor physical, functional, infrastructure, transportation services, environmental, and health conditions (Abbaszadegan, 2010)(Assadpour & Melles, 2018).

According to the Urban Renewal Organization of Tehran (2016), 5% (3268 hectares) and 24% (14792 hectares) of the city include distressed and unsustainable areas, respectively. The areas suffer from the lack of infrastructure, unstable and left buildings, fine-grained parcels inappropriate for redevelopment, the concentration of poverty, crime and disorders, and unsafe spaces. Tehran Municipality and the Organization for Urban Redevelopment have implemented several plans and renewal projects to address the deterioration of the built environment during the 1990s and 2000s. Following the infill development approach, the renewal and regeneration projects have aimed to revitalize the inner urban areas and prevent the continuation of urban decay. One of the strategies applied in this regard in Tehran has been urban catalysts (Sajadzadeh et al., 2018) (Hashemi et al., 2020). The most common projects include building malls, markets, hotels, cultural or community centers, parks, streets, public spaces, walkways, parking, etc. However, there is still not a clear border between catalysts and non-catalysts projects in Tehran. In other words, calling a project a catalytic one primarily refers to the urban authorities and their developers' efforts to highlight the benefits of a project in their surrounding areas. The majority of the projects have been funded by the government/municipality (public sector) or through the public-private partnership model. Public-private Partnership (PPP) as a tool to overcome the financial crisis and the low performance in public administration (Codecasa & Ponzini, 2011) has been largely recommended and applied in urban regeneration (Kort & Klijin, 2011), (Mohammadi et al., 2014) and can be seen as one of the manifestations of neoliberalism. The neoliberal perspective in urban regeneration has also been accompanied by deregulation, competitiveness, and stakeholder engagement. Negotiation with developers, determining privileges for partnership, and lowering the building laws to make the collaboration attractive and efficient for developers are among the strategies Tehran Municipality has applied in urban regeneration.

While urban catalysts have been accepted as a valuable tool in urban regeneration by many practitioners and scholars, one can hardly find an unambiguous definition for urban catalysts. In other words, there is still not a clear definition explaining what the tools should bring with them and how they should influence their surroundings. Therefore, this paper is going to provide a more comprehensive definition of the tools by reviewing the existing literature. Moreover, by looking through the negative consequences or inefficiency of such projects and practices, it tries to set the criteria through which we can reach more sustainable urban catalysts.

The research aims to provide a more comprehensive and clearer definition of urban catalysts in practice and in the shadow of the sustainable development concept. Firstly, as a result of the literature review, it was discussed that if urban projects are creating changes in the everyday activities in an area, the general image of the area, the physics and texture, and economic value of a built area could be considered as a project with catalytic impacts. However, the catalytic consequences could be valuable if those could be screened through seven criteria. Thus, as the second step, the authors asked a group of experts in sociology, civil engineering, urban planning, architecture, environmental specialist, economist, and management to list the sub-criteria and indicators for each criterion considering the particular conditions in the City of Tehran. The lists were combined, integrated, checked, and recategorized at a meeting with experts at Tehran Municipality. Therefore, the list has been used as a checklist for evaluating and ranking the case studies in Tehran (Table 2).

The research strategy was a case study strategy, and for that, as the third step, sixteen projects were chosen and analyzed through a checklist method. The case studies were selected from the suggestions of various experts at Tehran Municipality, practitioners, and academics in Tehran. In the fourth step, the authors collected and reviewed the published and official reports like newspapers, journal articles, documents. They made a summary of all critical and positive notions and impacts reported regarding the projects. Then, the projects were evaluated and ranked through expert judgment (a group of experts of different specialties and fields scored each project on each indicator based on a Likert spectrum (from 1 to 5). Finally, the scores of each project were summed up, and the analytical comparison diagrams have been made.

To improve the validity and credibility of the results, the authors checked the preliminary results of the evaluation process with experts at Tehran Municipality to consider any necessary change.

To understand how different types of projects respond to the criteria, various kinds of recent projects and practices in Tehran were chosen to be scored through the checklist prepared based on Table 2. The projects were chosen based on the comments of a group of experts who were familiar with the recent projects in Tehran or were engaged in such projects at the Tehran Municipality. The projects were also selected based on their significant reported or expected impacts on their surroundings (neighborhood, city region, or city). The projects include a range of large-scale projects and flagships to non-physical practices (Table 3).

The results, illustrated inFigure 3, show that nearly all projects achieve the lowest scores from the market adaptability criterion. However, the more sensitive projects to their context (context-based development criteria) and the social quality of life got the highest scores.

The large-scale projects show that despite the expected economic and social benefits of the development project of Persian Gulf Artificial Lake in the explanatory reports of Tehran Municipality, the project got low scores in market adaptability and economic prosperity criteria. Due to the new character, the project has made the City and the western regions, and tourist attraction and physical branding achieve a high score. However, the project did not get a good score in the environmental aspect because of its insufficient attention to the surrounding neighborhoods' health quality and environmental protection. Both the development projects of Nahjolbalaghe and Velayat Parks got high scores in the environmental aspect. Velayet park got a high score in social sustainability, pattern-making, branding, and contextual sensitivity. The project also includes various activities and has provided various job opportunities for different groups giving it higher scores in the economic prosperity criterion. Besides, the project has been developed in a low-income neighborhood with low access to such entertainment and green facilities. The project's good access to public transportation is another reason the project got a high score compared to the other recent projects and practices in Tehran. On the other hand, Nahjolbalaghe park was developed in an unsafe and brownfield area and was designed as a variation of well-designed open spaces for different users and activities. The unique landscape design of the project also causes the project to get a high score in terms of branding.

As it was mentioned in the problem statement and the literature, megaprojects and large-scale projects have usually been criticized due to their social and economic consequences (Flyvbjerg, 2012),(Gellert & Lynch, 2003), (Marrewijk et al., 2008)). The cases studied have been developed by public budget, and two of them (Velayat Park, and Nahjolbalaghe park) targeted the leftover lands (a formerly military airport and a valley with substantial social and environmental problems). Therefore, they had a considerable role in the city improvement, among other things. However, the Persian Gulf Artifical Lake is mainly designed to make a new brand for the city and the region to stimulate and serve the high-density housing and office developments in region 22 of Tehran (a newly built region).

Regarding the flagship projects, based on the research findings, the development of Tabi'at Bridge, Khavarn cultural center, and Bahman cultural center have successfully improved social quality. At the same time, the shopping malls, Milad Tower, and the Grand Mosalla of Tehran were not successful in the social sustainability criterion. However, The Grand Mosalla of Tehran, Milad Tower, and Tabi'at Bridge got high scores in branding since those work as contemporary landmarks of Tehran now. Also, the flagship projects instead of the Hyperstar shopping mall and Koroush shopping mall got high scores in branding and pattern-making. The unique architectural form of the Tabi'at Bridge, reflecting both natural and modern features, its adaptation with the natural areas surrounding it, and the provision of a vibrant public space, causes it to get high scores in the sensitivity to context, pattern-making, branding, and social sustainability. Bahman and Khavaran cultural centers got high scores due to the functional variation, job production, and influence on the social improvement of the surrounding communities.

As discussed by (Temelova, 2007), Healy et al. (1992), and (Loftman & Nevin, 1995), most of the flagships are mainly justifiable in economic terms and locate in profitable locations. It is noteworthy that except Khavaran and Bahman cultural centers, the other flagships evaluated in this study have been located in the medium to high-income regions. Most of the projects, especially the shopping malls, which have been developed by the private sector, were developed with a higher density or other land uses than was set by building by law and the city master plan. Even though it could be argued that the projects are attracting people and inviting more developers and activities to their surroundings, the projects can be criticized due to their role in segregation and the exacerbation of the gap between the lower and higher-income neighborhoods ((Temelova, 2007); (Cultural Policy and Urban Regeneration: the West European Experience, 1993)., Harvey, 2000).

Among the urban design and non-physical projects, the transformation of Hefdah-e-Shahrivar got a low score because of the dissatisfaction of a group of people, lack of public participation, and inadequate attention to other criteria. The high score of Panzdah-e-Khordad street and Marvi Gozar are because the projects aimed to limit the physical interventions in the existing structures and were appreciated as almost successful rehabilitation and regeneration projects. However, inadequate attention has been paid to market adaptability, economic prosperity, and environmental sustainability in these projects. Among the other non-physical projects/practices, the Qasr prison land-use change to a museum got a good score. At the same time, it has not considered the environmental and economic criteria effectively.

As it was discussed in the literature chapter, the most recent strategies recommended for urban regeneration include smaller-scale, not expensive, and non-physical changes (Oswalt et al., 2013),(Martin & Hincks, 2020),(Hashemi et al., 2020)). The projects, especially those designed or led by communities and the citizens' initiatives, can be successful in terms of social sustainability. Tactical urbanism and temporary use, and urban acupuncture can be useful tools for urban regeneration. However, the studied cases show that the projects' success also depends on the quality of community engagement and the continuity of the projects/practices. Their catalytic impacts are also dependent on the other complementary projects/practices supporting their positive impacts in the regeneration of a degraded urban texture. In other words, as it was mentioned, the catalysts are strategic tools (Bohannon, 2004),(Attoe & Logan, 1989), and their effectiveness relies on the following and complementary policies and actions.

The scores the selected projects in Tehran have gotten in each sub-criteria and indicator can present an overview of the common approaches of city projects and practices in Tehran. For example, in the environmental sustainability criterion, inadequate attention has been paid to the betterment of public access to the projects, energy-saving, sustainable energy resources, sustainable construction materials, recycling, reducing human heat, and reducing water and soil pollution. Regarding sensitivity to context, the visual adaptability of the project structure with the surrounding physical and natural features, adaptation with users' demands and comfort, acceptable walking distance, and protection and conservation of the existing natural and human-made elements, particularly the valuable and stable ones have been very effective in the higher or lower scores. Insufficient attention to the vernacular, local, and environmental-friendly values and architecture, encouraging social interactions, and not having a strategic perspective on physical development and placemaking (in pattern-making and branding criteria) are also seen in the recent projects.

The lack of evaluation criteria for evaluating physical and non-physical approaches causes the authorities to decide based on their attitudes. Developers can get the municipality's support by distorting their projects' attributes and exaggerating their projects' benefits. Although personal attitudes can also influence the qualitative scoring of a project in different criteria, such criteria can lead to more projects toward better quality in the long term.

Figure 4 suggests a framework for the evaluation of any catalytic projects. It aims to screen the projects' impacts and guide planners, authorities, and developers in designing, locating, and approving urban catalysts for urban regeneration. The framework has been made up of seven primary questions addressing the seven criteria for evaluating the projects toward sustainability and the four dimensions of catalyst projects adopted from the literature review and discussion.

Urban Catalyst is a relative and comparative term. However, it can be a seductive and precarious title. How such projects are valued is strongly dependent on society's values, including the values determined directly or indirectly by those in power. According to the literature review, such projects usually affect an urban texture in four dimensions, including reimagining, activity generation, physical stimulation, and economic value creation. These aspects get different meanings in different spatial and temporal realms along with the societal, technical, and value changes. Therefore, identifying such values and considering those as the basis for evaluation of the projects' impacts is significant in order to guide and direct the future pioneering project.

This paper discussed the likely conflicts between the catalytic projects' impacts (the four aspects) and the sustainability goals (economic, social, environmental, and physical sustainability) through the existing literature. According to the conflicts, it suggests an evaluation framework, including seven criteria for evaluating catalytic projects. The criteria include physical and content identification, pattern making, contextualism, environmental sustainability, market adaptability, economic prosperity, and improvement of social quality of life. The criteria are explained as the questions which could be translated based on each city context and the significant values considered by each community. The framework can be used either in the preliminary process (evaluation of proposals) or when the projects or practices are done and are the subject of evaluation due to other reasons like receiving financial aid or other credits and privileges.